The semiconductor industry is facing increasing pressure and challenges to continually shrink the dimensions of semiconductor devices. The conventional top-down method of fabricating defect-free, arbitrary shaped, ultra-small semi-conducting structures and devices has its limitations. The top-down method is largely an extension of micro-fabrication methods used for fabricating single crystalline structures.
The single crystalline structures are the basic building blocks of micro-technological and nano-technological systems, including transistors, Field Effect Transistors (FETs), sensors, Micro-Electro-Mechanical Systems (MEMS) devices and light emitting solid-state devices. Typically, a single crystalline structure is fabricated monolithically on a single crystal substrate using lithographic techniques and wet/dry etching processes in combination with epitaxial growth techniques such as Solid Phase Epitaxy (SPE), Liquid Phase Epitaxy (LPE), Chemical Vapor Deposition (CVD), Molecular Beam Epitaxy (MBE) and Metal-Organic Vapor-Phase Epitaxy (MOVPE).
The Vapor-Liquid-Solid (VLS) method is an alternative technology to the previously mentioned techniques that facilitate growth of epitaxial whiskers. An epitaxial whisker is a hair-line structure grown epitaxially. In the VLS method, a metal particle is used as a seed. The seed particle, which can have catalytic properties, is heated in an environment containing vapors of gaseous precursor molecules along with optionally a semi-conducting material. Due to heating, a eutectic melt is formed in the seed particle. When the semi-conducting material in the seed particle reaches a critical saturation concentration, the semi-conducting material precipitates out of the seed particle in the form of a circular shaped structure with the same crystallographic orientation as the substrate.
The principle of the VLS method may also allow growth of epitaxial whiskers in a liquid environment. In this case, the gas phase is replaced by a liquid phase and the method is then called Liquid-Liquid-Solid (LLS) technique. Other family members of the VLS technique are Vapor-Solid-Solid (VSS) and Super-critical Liquid-Solid-Solid (LSS) techniques that differ from LLS because the seed particles remain in the solid state as opposed to a liquid state. Therefore, VLS is hereinafter used as synonym for LLS, VSS, and LSS.
It is known in the art that the shape and the position of the seed particle deposited on the substrate may change during annealing due to either uptake of substrate material, or formation of alloys with the adsorbed material from the gas or liquid phase. The shape and the position of the seed particle deposited on the substrate may also change due to annealing or surface diffusion. Further, the material properties of the structures produced using solid seed particles are inferior compared to the structures grown using liquid seed particles. The former structures exhibit more growth defects and have a reduced epitaxial relationship with the substrate. Further, the solid seed particles have generally a reduced efficiency to adsorb and catalyze precursor molecules and uptake material from the vapor or liquid environment.
FIG. 1a illustrates seed particles 102, 104, 106, 108 and 110 that act as seeds for crystal formation and are present on a substrate 112. It can be seen from the figure that seed particles 102, 104, 106, 108 and 110 present on substrate 112 are arbitrary-shaped. FIG. 1b depicts the top view of substrate 112 with seed particles 102, 104, 106, 108 and 110.
FIG. 1c illustrates the shape of seed particles 102, 104, 106, 108, 110a and 110b after the particles are annealed at eutectic temperature. The particles melt at eutectic temperature acquiring a circular shape. FIG. 1d depicts the top view of substrate 112 with seed particles 102, 104, 106, 108, 110a and 110b after the annealing process. It may be seen from the figure that after annealing, square-shaped seed particles 108 and 110 in FIG. 1a and FIG. 1b which were square shaped intentionally, become circular or drop-like due to the breaking up of the film. The film is broken up in small islands and droplets because of surface tension of seed particle 110 that reduces the exposed surface area of the seed particle forming seed particles 110a and 110b. 
FIG. 1e illustrates substrate 112 with crystals 122, 124, 126, 128, 130 and 132 grown under seed particles 102, 104, 106, 108, 110a and 110b respectively. FIG. 1f depicts the top view of substrate 112 with crystals grown under the seed particles. It is apparent that the cross-section of the resulting crystals is limited to a circular shape since the liquefied seed particles acquire a circular shape during the growth process.
Thus, the VLS technology allows fabrication of defect-free, ultra-small semi-conducting structures that have essentially a circular-like shape (including for examples triangular or polygon like shapes that approximate a circular shape) only. Therefore, there is a need to devise a solution for growing crystalline structures of high quality which have a pre-defined cross-section. There is also a need to improve the smoothness of the structures by avoiding process damages induced during traditional micro-fabrication steps.